1. Field of the Invention
The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube with a thin film having light selectivity and on optical filter being formed on the front surface of a faceplate of the color cathode ray tube.
2. Description of the Related Art
In a color cathode ray tube, electron beams from an electron gun assembly arranged in a neck of an envelope are bombarded on a dot or stripe of red, green, and blue emitting phosphor layers regularly formed on the inner surface of the glass faceplate, thereby displaying characters and/or images.
A red emitting phosphor in this color cathode ray tube generally consists of europium-activated yttrium oxide (Y.sub.2 O.sub.3 :Eu) or europium-activated yttrium oxysulfide (Y.sub.2 O.sub.2 S:Eu). Although the Y.sub.2 O.sub.2 S:Eu phosphor can provide redness to some extent by color correction using an Eu activator concentration, sufficient brightness as a red pixel of a color cathode ray tube cannot be obtained.
Since the Y.sub.2 O.sub.2 S:Eu phosphor does not have satisfactory temperature characteristics, its brightness is lowered with an increase in temperature of a faceplate upon electron beam radiation. In order to explain this relationship, a relationship between the electron beam radiation time and the brightness of the red emitting phosphor is plotted in a graph of FIG. 1. As shown in FIG. 1, when an electron beam of 10.4 .mu.s/cm.sup.2 impinges on the Y.sub.2 O.sub.2 S:Eu phosphor, the brightness of the phosphor is lowered by about 8% in 120 sec. After a lapse of 120 sec. or more, the brightness is gradually lowered. The Y.sub.2 O.sub.2 S:Eu phosphor does not have satisfactory current-brightness characteristics. That is, when a current density is increased, a decrease in brightness tends to be increased. In particular, a red emitting phosphor has a higher current ratio than that of a blue or green emitting phosphor. Therefore, when the current-brightness characteristics of the red emitting phosphor are not sufficient, a serious problem is posed.
To the contrary, the Y.sub.2 O.sub.3 :Eu phosphor has a very high emission brightness level as compared with the Y.sub.2 O.sub.2 S:Eu phosphor and satisfactory temperature characteristics, as shown in FIG. 1. FIG. 2 is a graph showing a relationship between the current density and the relative brightness of the Y.sub.2 O.sub.3 :Eu phosphor for various Eu activation amounts when the brightness of the Y.sub.2 O.sub.2 S:Eu phosphor is given as 100%. As is apparent from FIG. 2, the relative brightness of the Y.sub.2 O.sub.3 :Eu phosphor as a function of an increase in current density is higher than that of the Y.sub.2 O.sub.2 S:Eu phosphor. Judging from this, it is understood that the Y.sub.2 O.sub.3 :Eu phosphor has satisfactory current-brightness characteristics. As shown in FIG. 2, even if an activation amount of Eu in the Y.sub.2 O.sub.3 :Eu phosphor is increased, brightness saturation rarely occurs. For this reason, the Y.sub.2 O.sub.3 :Eu phosphor has a higher brightness level in a large-current range, thus providing satisfactory phosphor properties. When an Eu activation amount is 4.5 mol% with respect to the base material, a practical color purity of a color cathode ray tube can be obtained. In this case, the Y.sub.2 O.sub.3 :Eu phosphor has a higher emission brightness level than that of the Y.sub.2 O.sub.2 S:Eu phosphor by +30%.
The Eu concentration is represented by an average molecular weight of the phosphor itself, i.e., {number of moles of Eu.sub.2 O.sub.3 contained in 1 mol).times.100} when it is figured out as an average molecular weight of a compound obtained by partially substituting Y of Y.sub.2 O.sub.3 with Eu.
Along with the recent development of a larger color cathode ray tube, performance of an electron gun assembly for emitting electron beams, and particularly its focusing capacity has been improved. It is expected that the performance of the Y.sub.2 O.sub.3 :Eu phosphor on the phosphor screen can be improved by suppression of brightness saturation, and that the capability of the high-performance electron gun assembly can be maximized. However, even if an Eu activation amount of the Y.sub.2 O.sub.3 :Eu phosphor is increased, a sufficient color purity cannot be obtained as compared with the Y.sub.2 O.sub.2 S:Eu phosphor. FIGS. 3a and 3b show the chromaticity coordinate values (y and x values) and the Eu activation amount of the Y.sub.2 O.sub.3 :Eu phosphor, respectively. Ranges indicated by a hatched region in FIGS. 3a and 3b, i.e., ranges of .times.=0.620 or more and y=0.345 or less, are practical chromaticity ranges of the Y.sub.2 O.sub.2 S:Eu phosphor. The corresponding Eu activation amount falls within the range of 3.0 mol% to 4.4 mol% with respect to the base material. As compared with the chromaticity ranges of the Y.sub.2 O.sub.2 S:Eu phoshpor, the chromaticity coordinate values of the Y.sub.2 O.sub.3 :Eu phosphor are x=0.628 and y=0.347, which are not practical. Even if an Eu activation amount is increased, changes in chromaticity are decreased with an increase in Eu concentration. Therefore, the y value as the chromaticity coordinate value does not reach the range represented by the hatched region. It is impossible to maintain image quality of the Y.sub.2 O.sub.3 :Eu phosphor to be equal to that of Y.sub.2 O.sub.2 S:Eu phosphor. A red emitting phosphor ideally has satisfactory brightness characteristics as those of the Y.sub.2 O.sub.3 :Eu phosphor and a satisfactory color purity as that of the Y.sub.2 O.sub.2 S:Eu phosphor at a low Eu activation amount.
In recent years, in order to improve color purity of a red emission component, suppress degradation of image brightness, and improve contrast, a color cathode ray tube having a neodymium oxide (Nd.sub.2 O.sub.3)-containing glass plate to obtain a selective light-absorbing property formed on the front surface of a faceplate has been proposed (Published Unexamined Japanese Patent Application Nos. 57-134848, 57-134849, and 57-134850). This glass plate has a narrow main absorption band in a range of 560 to 615 nm and a sub absorption band in a range of 490 to 545 nm due to light-absorbing properties inherent to neodium oxide. Therefore, red and blue color purity values of an image can be advantageously increased.
Although this glass plate has selective light-absorbing properties, the contrast cannot be greatly improved. A method of evaluating an effect of contract improvement using BCP (Brightness Contrast Performance) is available. This BCP is defined as BCP=.DELTA.B/.DELTA.Rf where .DELTA.B is the brightness decrease rate and ARf is the rate of decrease in reflectance of ambient light. The BCP represents a contrast improvement ratio when a system using a neutral filter is assumed as a reference. When a neodium oxide filter having selective light-absorbing properties is evaluated by using the BCP, the BCP falls within the range of 1.ltoreq.BCP.ltoreq.1.05. It is therefore understood that the contrast is not sufficiently improved Since the glass plate containing neodium in the main absorption band of 560 to 570 nm in a wavelength range of 560 to 615 nm, the color (body color) of the glass plate itself is changed by ambient light. In particular, the body color of the glass plate under an incandescent lamp becomes reddish. For this reason, a low-brightness portion such as a black or shadow portion in an image becomes reddish, readability is degraded, and image quality is degraded. In addition, since neodium is an expensive material, the resultant glass plate becomes expensive.